The present invention relates to a tribological test apparatus, and in particular to an apparatus for performing contact surface fatigue tests by subjecting the surface of disk-shaped specimens to cyclic Herzian stress, i.e. cyclic contact pressure.
As is known, tribology studies the interaction, and in particular friction, wear, and lubrication phenomena, of surfaces of contacting bodies under load and in relative motion. In tribology, a need is particularly felt to determine the life of machine components subjected to rolling contact wear caused by cyclic mechanical stress of a surface in rolling contact with at least one other body; which stress, repeated over time, induces fatigue damage phenomena, which are manifested by the removal of fragments of material from the surface involved. Rolling contact wear is one of the main factors responsible for the failure of components such as rolling bearings, gears, and wheel-rail couplings, and is fairly difficult to detect at the initial stages, on account of initial fatigue (particularly in the case of surface hardened components) producing cracks beneath the outer contact surface.
When sufficiently widespread, the cracks emerge on the outer contact surface, and may result in splintering and, therefore, in immediate impairment in component efficiency, caused by a drastic change in component geometry.
The process defined by cyclic stress resulting in component wear and, eventually, failure as described above is known as xe2x80x9crolling contact fatiguexe2x80x9d, and the above surface damage as xe2x80x9cpittingxe2x80x9d.
Pitting phenomena can be divided into two classes: initial or micropitting compatible with functioning of the component; and destructive or macropitting marked by the formation of macroscopic damage.
To determine the life of components subjected to in-service rolling contact fatigue, apparatuses are known for laboratory testing disk-shaped specimens made of the same material (both the base material and any surface treatment) as the study component: the specimens are rolled mutually, and the number of Hertzian contact load cycles to which the specimen surfaces are subjected upon the onset of macropitting phenomena in the specimens is recorded.
Such known apparatuses can be divided into two types, a first of which comprises two specimens rotated at variable, independent speeds and pushed one against the other by a variable load. The asymmetry of the system and the single contact region between the surfaces obviously limit the loads that can be applied, and greatly increase test time. For this reason, a second type of apparatus has been proposed comprising four specimensxe2x80x94one central and three peripheralxe2x80x94which are fitted in fixed positions to respective parallel shafts rotating about respective axes. The peripheral specimens are spaced 120xc2x0 apart about and in contact with the central specimen, and are pushed radially and simultaneously against the central specimen as they are rotated. The peripheral specimens are fitted to and project from relatively long transmission shafts, the flexural elasticity of which enables the peripheral specimens, when subjected to radial thrust, to oscillate radially by relatively small amounts (about a tenth of a millimeter) and so exert pressure on the central specimen as they roll mutually. Three Hertzian contact load cycles are thus performed on the central specimen at each turn.
Radial thrust is applied on the peripheral specimens using pressurized oil, which also provides for lubrication and is fed into a radial gap between each peripheral specimen and a relative C-shaped body fixed adjacent to the contact surface of the peripheral specimen on the diametrically opposite side to the central specimen.
The presence of pitting phenomena is determined using an accelerometer fitted to the central specimen shaft, and the readings of which are used to calculate the power spectrum integral, i.e. energy, associated with the main frequency harmonics of the shaft. When the energy calculated exceeds a given (experimentally determined) threshold indicating the onset of pitting phenomena, testing is stopped to avoid subjecting the apparatus to dangerous vibration caused by surface damage to, and a variation in the geometry of, the specimens. Obviously, the total number of load cycles withstood by the central specimen (three times that of the peripheral specimens) is the significant value by which to compare tests and assess the specimen materials.
The above known apparatuses are unsatisfactory by employing oil, necessarily suitable for use in hydraulic systems, to apply the peripheral to central specimen contact load, and therefore cannot be operated for tribological testing using just any lubricant, and in particular the actual in-service lubricants of the mechanical parts for testing.
Moreover, known apparatuses of the above type are extremely complex, and therefore expensive, to produce and assemble. That is, the specimens and C-shaped bodies must be machined and positioned extremely accurately to ensure the relative oil gaps are all the same size and as uniform as possible during testing, and so ensure the oil flow acting directly on the peripheral specimens exerts perfectly balanced thrusts in perfectly radial directions with respect to the central specimen.
Once the geometry of the apparatus is defined, the specimens cannot normally be replaced with others of different shape and/or size, which would also mean replacing the C-shaped bodies and shaft bearings to adapt to the size and shape of the new peripheral specimens.
Moreover, when the peripheral specimens are subjected to thrust by the pressurized oil, the transmission shafts flex elastically towards the central specimen as they rotate, and so undergo combined rotation and bending fatigue stress which tends to reduce their working life.
Known apparatuses of the above type cannot normally be used to simply determine surface wear of the specimens, owing to the extremely small maximum travel of the peripheral specimens to and from the central specimen, so that, when wear exceeds the maximum travel, the peripheral specimens can no longer be brought into contact with the central specimen.
Finally, to determine the instant marking the onset of pitting phenomena, a complex, high-cost computing and control device is required to stop testing immediately pitting phenomena arise.
It is an object of the present invention to provide a tribological test apparatus designed to provide a straightforward, low-cost solution to the drawbacks of the aforementioned known apparatuses.
According to the present invention, there is provided a tribological test apparatus comprising:
a supporting structure;
a central shaft extending along a first axis, connected to said supporting structure to rotate about said first axis, and supporting a central specimen;
at least three peripheral shafts extending along respective second axes equally spaced about said first axis, connected to said supporting structure so as each to rotate about the respective second axis, and supporting respective peripheral specimens radially facing said central specimen;
push means for exerting on each said peripheral specimen a force directed towards said central specimen; and
relatively mobile means associated with said peripheral shafts to enable each said peripheral specimen, in use, to move radially towards said central specimen and exert a contact-pressure on said central specimen under the action of said force;
characterized in that said push means comprise, for each said peripheral specimen, a movable push member for exerting said force along a push axis intersecting said first and said second axis;
Said relatively mobile means preferably comprise, for each said peripheral shaft, guide and slide means enabling said peripheral specimen to translate between a radial contact position radially contacting said central specimen, and a radially detached position radially detached from said central specimen; transmission means being provided for rotating each said peripheral shaft independently of its translation.